%intro
\section{Technologies}
To connect different things to the IoT, adequate technologies are necessary. These technologies depend on the individual application area. 
Due to this distinctness of ''things'', different technologies are necessary to connect all of them. For example, cars and roadsigns cannot communicate by using the same technology as products in a distribution center (i.e. supply chain). Thus, a few different technologies are discussed below that are able to cover almost every considerable ''thing''. 

\subsection{Internet Protocol}
The term {IoT} itself implies that a technology is necessary for connecting the ''things'' that is comparable to the actually used Internet Protocol (IP). The IP is a well-proven technology which is the basic technology of today's Internet. However, in the past few years the IP (i.e. more exactly IPv4) reaches its limit due to the limited address space. If every ''thing'' should be connected to the Internet it would not be possible to assign an IP address to every device. Additional techniques like {NAT} have to be used in this case to save public IP addresses. 

With the introduction of IPv6, the opportunities for a worldwide IoT were increased dramatically. Due to the enormous address space of 128 Bit, it is now possible to assign an IP address to almost everything. Additionally, the old IPv4 header was revised to save unnecessary data. Furthermore, the IPv6 standard directly contains security services to guarantee privacy, authenticity and confidentiality as well. These mayor improvements of the IPv6 in contrast to the outdated IPv4 provide a basis for the upcoming communication.

\subsection{Wireless Personal Area Networks}
In the previous subsections IP as a technology for the worldwide decentralized communication was introduced. However, IPv6 as well as IPv4 normally base on Ethernet (IEEE 802.3). Since most of the things (e.g. in a household) are mobile, wired technologies are infeasible for such things. Thus, wireless systems are necessary to connect all the things to the Internet. 
\begin{itemize}
\item
The first considerable technology is WirelessLAN (IEEE 802.11) which provides a wireless basis for IPv4 as well as IPv6. WirelessLAN operates in the 2.4~GHz industrial, scientific and medical (ISM) frequency band and has a range of about 100 meters. The default topology of WirelessLAN is the so called \emph{star-topology} as shown in Figure~\ref{fig:topos}b where every node of the network has to be connected to the center point (e.g. the gateway). The center point of the wireless network is then connected to a wired network. Furthermore, a \emph{point-to-point} (cf. Figure~\ref{fig:topos}a) connection is possible where two single nodes are connected with each other. By using this technology, it is not possible to use the routing mechanisms of an upper layer protocol such as IP. Therefore, a multi-hop communication as shown in Figure~\ref{fig:topos}c cannot be established. 

A few month ago, an addendum to WirelessLAN was released (IEEE 802.11s, c.f. \cite{4597114}) which enables multi-hop structures for wireless networks. However, since the addendum was presented recently, no such devices exist up to now. 

When WirelessLAN was introduced, the intended use case was the wireless connection of PCs and Laptops in homes and offices. In this use case, properties like energy consumption and performance regarding embedded devices played a minor role. However, in the world if IoT, these properties are very important, since most of the devices are battery-powered and have limited hardware resources. Thus, WirelessLAN seems to be a poor technology for the use in the IoT. 

\item Another promising wireless technology in the context of IoT is ZigBee \cite{Akyildiz2005445}. ZigBee is a wireless communication protocol founded by the ZigBee Alliance. 
ZigBee is based on the IEEE 802.15.4 standard which specifies the physical and the datalink layer of ZigBee. IEEE 802.15.4 specifies different frequency bands: two in the Sub-GHz area around 900~MHz and one in the 2.4~GHz band such as WirelessLAN. The range in the Sub-GHz bands is about 400 meters and the range in the 2.4 GHz band is similar to WirelessLAN (i.e. 100 meters). 
This technology is intended to be used for wireless control networks in home and industrial environments. The data bandwidth of ZigBee is far below those of WirelessLAN, but in this domain it is not that important to transmit high amounts of data. ZigBee is also a very lightweight protocol which makes the usage in low-cost embedded systems feasible. Due to the lightweight fashion of ZigBee, it is also feasible for low-power applications in battery-powered devices. One of the mayor advantages of ZigBee is the multi-hop capability. Thus, it is possible to connect devices which are not directly connected to the gateway as shown in Figure~\ref{fig:topos}c, since almost every device is able to act as router.

Unfortunately, the standard does not define how to transmit or handle IP messages. One possibility to overcome this issue is to assign an IP address to every ZigBee device and encapsulate the IP packets in ZigBee messages. Another possibility is to use a gateway to connect the ZigBee network to the worldwide IP network. \cite{2010arXiv1002}

\item
6loWPAN as introduced in \cite{Mulligan:2007:ARC:1278972.1278992} tries to overcome the issues of the two previously mentioned technologies. On the one hand side, the IEEE 802.15.4 standard is used for the physical communication link. This protocol guarantees a lightweight low-power and low-cost communication platform. On the other hand side, it was tried to setup the IPv6 protocol above the IEEE 802.15.4 Data Link Layer (DLL). 

As mentioned before, the multi-hop ability is an important property of a WPAN technology. 6loWPAN provides two different mechanisms to handle this kind of routing. On the one hand side it is possible to use broadcast messages on the DLL. If such a message is received by a device, it forwards the message immediately. However, due to the high amount of data on the network, transmissions can be disturbed or delayed. Fortunately, the IP was originally designed to deal with such conditions. On the other hand, a dedicated routing algorithm is used like in wired IP networks. 

%Thus, 6loWPAN is the most promising technology of WPANs in the context of IoT.

\end{itemize}


\begin{figure}[h]
	\centering
		\includegraphics[width=1\columnwidth]{fig/concept.pdf}
	\caption{Wireless network topologies}
	\label{fig:topos}
\end{figure}

\subsection{Radio-Frequency Identification}
The Radio-Frequency Identification (RFID) technology as introduced in \cite{977758} is on the lowest end of the communication hierarchy. The system is used to identify things such as it is done by simple barcodes. In case of barcodes the object of interest is identified by a special kind of image (i.e. the barcode) which is scanned by an optical system. By using RFID, the object is tagged with a small so called {RFID}-Transponder. This transponder consists of a microcontroller and a radio which are powered by induction through the built-in antenna. If an RFID-Tag should be identified, an RFID-Reader can request information by sending an electromagnetic signal. The RFID transponder can use this electromagnetic signal as energy source and is able to harvest enough energy to transmit its unique ID to the {RFID}-Reader. In addition to a unique ID, it is also possible that sensor data are transmitted (e.g. temperature) and therefore RFID-Transponders can also be used as sensors.

The mayor advantage of such a tag is that the transponder is completely passive and can be deeply integrated such that the size of a transponder is less than a centimeter. Furthermore, the costs of one tag are so small that nearly every ''thing'' can be tagged. Unfortunately, the range of such a system is very small (i.e. around 3 meters) such that a reader nearby the tag is necessary. Obviously, a communication between two different RFID-Transponders is also not possible.

As mentioned before, the RFID technology was intended to be used for tracking and identifying goods or products. In this context, the RFID technology is used for the {EPC} as stated in the introduction. It standardizes the worldwide identification of goods and products such as the classical barcode nowadays. An {EPC} ID consists of at least 64 Bit, but it is possible to extend this ID up to 204 Bit. 

